Astronomers find parent star of lonely planet

Twinkling stars may be the stuff of nursery rhymes so it's fitting that the latest astronomical discovery reunites a lonely planet with its parent star a trillion kilometres away.

Dr Simon Murphy from the Australian National University Research School of Astronomy and Astrophysics and a team of international scientists were studying a gas giant planet known as 2MASS J2126−8140 when they discovered it was in orbit with a small red dwarf star, albeit at quite a distance.

"There's been a handful of wide planet-like systems discovered in the last five or six years, but this is by far the widest."

The study was part of a survey of several thousand young stars and brown dwarfs.

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Dr Murphy believes more pairs may soon be discovered now scientists know what to look for.

"We stumbled on this one, both of these objects have been known for a while we were just the first to tie them together," he said.

"No study had looked wide enough around to find the other star.

"This is the first really exciting one we've found."

Dr Murphy said the planet was somewhere between 11 to 15 times the mass of Jupiter placing it "on the fuzzy boundary" between planet and brown dwarf status – an object too large to be a planet and too small to be a star.

Previously scientists had thought the planet was a member of a young group of stars near to the sun, but have now realised it couldn't have formed in the same way as planets in our solar system because of the vast distance from its host star known as TYC 9486-927-1.

The distance between the pair is 6900 Astronomical Units (AU) – 1,000,000,000,000 kilometres or 0.1 light years – nearly three times the previous widest pair, which is 2500AU (370,000,000,000 km).

Light would take about a month to reach the planet.

"When we look at young stars … they're usually surrounded by a disk of gas and dust from which the planets form, but it's rare to find a disk even a tenth as big as the separation between this star and planet," he said.

Dr Murphy said the team were now trying to establish if the objects were formed together, from a filament of gas or a molecular cloud with the vast distance in between, or separately finding each other while they were drifting away.

"Or the planet could have been a lot closer and some interaction with another star or a gas cloud disrupted the system and we're watching the aftermath of the hit and run as the planet slowly drifts away from its star," he said.

Dr Murphy said tying the giant planet to the star would make it easier for scientists to calculate its age and mass.

"Because it's very nearby it should be a good system for understanding how giant planets form and evolve," he said.